To test you can move a specific plugin to a .bak, then try totem on the file. If it then plays you're affected. The moving of the plugin is NOT a solution though many have used it as a temp workaround.

Most of the time, when you open up the video file in a player in Ubuntu, there must come a pop up message telling you - you are unable to watch the video, and then instruct you to install the right extension (Third party) .Mp4 plug-in so you will be able to watch that video

If you are not getting that, you can also look for a specific extension / plug-in that can do the job. Otherwise try installing VLC Media player, most time VLC can play almost everything....

This error could be caused by required additional software packages which are missing or not installable. Furthermore there could be a conflict between software packages which are not allowed to be installed at the same time."